Propellers or related rotating forms such as rotors, fans, and screws are an integral aspect in the design of boats and aircraft. Present watercraft screws are of low efficiency, having high noise signatures and are limited by water cavitating effects on blades. Their lack of safety with impact on manatees, people in the water, and submerged objects is frequently reported in the daily press. Proper stopping of small watercraft moving at 60 mph is questionable; no practical braking system presently exists to help avoid running into other crafts, swimmers, and debris. Large propellers rotating at slow speeds are a necessary ship design selection and, because of ship/screw size relationships, have marked flow interferences from the ship body resulting in a lower efficiency.
Airplane propellers and helicopter tail rotors, to have acceptable efficiencies, must move large volumes of air with a relatively low induced acceleration of the flow. The sizes of such rotors are relatively limited; therefore, to provide sufficient thrust, these are driven with high blade tip velocities. Tip vortices produced by high thrust/tip speed conditions add to the noise of the system.
Therefore, there is a need for a structural system which improves performance from propellers by enhancing their thrust, horsepower, and reducing their noise. A need also exists for a cage, which provides safety from impact by the propeller on inert or living objects, and protects the propeller without imposing a penalty on the performance of the propeller. A need also exists to reduce noise levels from the operation of a propeller used on watercraft, aircraft, helicopters and fans.
Previous patents were researched in the fields of propellers, screws, safety devices and for means of increasing propeller thrust with the following results:
U.S. Pat. No. 4,689,026 to M. S. Small relates to propeller screws and shows a square tunnel enclosing a screw which system would have excessive water drag and low flow efficiency because of the tunnel corners plus turning losses of the water. PA1 U.S. Pat. No. 4,666,411 is for a tubular, ship-wake generator which is also a high drag non-propulsive item. PA1 U.S. Pat. No. 4,031,846 to V. W. Tone describes a guiding drive and conical semi-cover which might act as an anti-ventilation plate but would definitely not be propulsive. As the conical plate covered only the tip of the propeller, it would not act as a safety guard. PA1 U.S. Pat. No. 3,742,895 describes a rudder in the propeller wake for steering and is not related to my invention. PA1 U.S. Pat. No. 3,528,382 describes a flow straightener to recover the rotational energy in the water wake by straightening the wake. This represents a small thrust gain at best in that wake velocity measurements indicate skew angles of flow to the shaft axis of rotation of less than 15.degree. at most. This would increase the thrust by 3.5% at most if no drag for the anti-rotation vanes existed; no appreciable positive thrust effects could be expected. PA1 U.S. Pat. No. 4,580,517 describes a shrouded propeller located in a tube which is a non-propulsive shroud system in forward motion. The tube, which in a static thrust condition improves the thrust, has such a high drag with speed as to force a large penalty in power required for motion. PA1 U.S. Pat. No. 3,722,454 shows a long, fluid tube adjacent to a propeller. The high skin friction of this device during forward motion precludes its use because of extreme power requirements at all but very low forward speeds. PA1 U.S. Pat. No. 4,441,163 shows a non-propulsive cage to protect the propeller. This cage, being non-propulsive in nature, causes excessive power to be required at any reasonable operating speed. PA1 U.S. Pat. No. 4,078,516 to D. C. Babus shows a non-propulsive cage again with the same high drag characteristics noted above. PA1 U.S. Pat. No. 3,968,944 to Freidheim Zimmer shows a tapered contracting shroud with a propeller located in the entrance. This again would present a high drag item as the shroud is non-propulsive in nature.
It may be noted that most of the above prior art describing shrouds or tunnels do protect personnel and/or sea creatures from propeller contact, but at the expense of efficient performance and with high power requirements because these are high-drag items.
My invention achieves the safety provided by a cage but with propulsive thrust augmentation even with reduced input power that can be as little as one-half of normal propeller power required to deliver the same speed.
A related U.S. Pat. No. 4,506,849, Helicopter Rotor Thrust Ring to H. E. Lemont, describes use of a single thrust ring around a tail rotor. Both crossflows from the main rotor and from translational flight are used to enhance thrust from the thrust ring. The area of the single thrust ring required to adopt it for sufficient axial propulsion augmentation would impose too much drag to operate successfully in an axial, high-speed mode.
Townend rings (an early method of streamlining radial aircraft engines) have been used to enclose aircraft propellers to augment thrust. This is an aircraft variant of the shrouds of U.S. Pat. Nos. 3,722,454 and 3,969,944 mentioned above. The same objections which apply to watercraft also apply to air applications. While these Townend rings boost static ground thrust, at moderate aircraft translational speeds drag soon equals the increased thrust effect to reduce the net effectiveness to zero.
Transport aircraft use propulsive fans which are included within a duct/cowling system for noise control. Thrust enhancement by the distribution of negative air pressures on the cowl lip as well as the elimination of the blade tip vortices when the fan blades are located in the duct are claimed. Variations of duct lip pressures with translational and cross flow cause changes in internal duct air velocities, both in direction and magnitude to effect internal fan face velocity distributions. This upsets the match of the required fan blade twist to the duct internal radial air flow distribution and reduces efficiency. To eliminate the tip vortices, the fan blade tips need to be very close to the duct wall (less than 0.5% of the radius) with subsequent clearance problems. The small clearance creates a major disadvantage because of the additional weight required to stiffen the duct structure to prevent contact by the fast-moving blades.
Patents that were cited as references against U.S. Pat. No. 4,506,849 use the principle of edgewise cross flow during translational flight velocity to achieve lift; thus, exhibiting similar limitations as noted for the '849 patent.